Replicate the light coming from a projector
I'm sure you used the search function and found how to make a projector spot lamp in cycles.

With this node setup you only need to add a cube with a Volume Scatter to create the volumetric lighting.




Use a highly saturated image.
Use a high contrast image.
If your image is neither, increase saturation and contrast with material nodes.
Use a low lamp size (0.0), otherwise your image will be blurry.
Use a strong lamp emission intensity (my value: 7000).
Use a small density of the volume scatter material.
Only encase the necessary areas with the volume scatter node to speed up rendering .